Magnetic resonance imaging (MRI) devices require a highly intense, homogeneous magnetic field of sufficient size to allow a portion of a patient to be placed in the homogeneous field and effectively visualized by MRI. MRI devices are valuable for medical diagnostic purposes, and are very instrumental in the early detection of cancerous tissues, tumors, and the like.
Generally, MRI systems operate by using a main coil of electrical windings to produce a magnetic field. A selected spherical region within the field is defined as the Diameter Spherical Volume (DSV). This is the selected design volume within which effective MRI images can be obtained. For MRI procedures, it is very important that the magnetic field in the DSV be sufficiently intense and homogeneous to provide proper image quality.
In most MRI systems, a hollow cylindrical main magnet is used, and the DSV is defined at a selected location, with a selected diameter, in the bore of the magnet. The patient can then be placed in the bore of the MRI system to position the particular portion of the patient's body to be imaged within the DSV. With the main coil activated, the magnetic field in the DSV is substantially homogeneous; in other words, there is a substantially uniform flux density throughout the DSV. This causes certain types of atoms in the patient, within the DSV, to effectively align their spin axes with each other. The patient is then momentarily subjected to radio frequency electromagnetic radiation from an RF source within the MRI device. The effect is to briefly tilt the atoms of the patient out of their original orientation and then allow them to return to that orientation, accompanied by the emission of RF waves which can be detected by an RF receiver. Measuring the intensity of the received signals as a function of the resonance frequency or magnetic field gives the operator information on the atoms of interest.
Additional electromagnetic coils, called gradient coils, are used to momentarily impose selected directional gradients on the main magnetic field within the DSV, to give the operator positional information on the atoms of interest. The gradient coils are selectively pulsed to impose these gradients while the patient is within the MRI device, to momentarily establish spatial identification within the DSV. Pulsing of the gradient coils coincides with the pulsing of the RF source. In other words, the gradient coils momentarily give each point within the DSV its own unique magnetic field value, establishing a unique resonance frequency for an atom at each point. Since the received RF signals are evaluated as a function of the resonance frequency, this enables the operator to identify the exact location of the source of each RF signal he receives as a result of reorienting of the spin axes of the atoms. Based on the position of the source and the intensity and frequency of each of the echo signals caused by the tilting and recovering of the atoms, the MRI system can produce an image of the portion of the patient positioned in the DSV.
Undesired inhomogeneities in the main magnetic field, however, will distort the position information established by the gradient coils and degrade the image quality. Where the radio frequency shifts of interest are in the range of a few parts per million or less, the inhomogeneity in the main magnetic field within the DSV must also be limited to a few parts per million. Unfortunately, even with the most meticulously designed and manufactured MRI devices, inhomogeneities in the main magnetic field will exist, and correction is necessary.
To improve field homogeneity, correction coils are frequently used. These correction coils, which are in addition to both the main coil and the gradient coils, are capable of creating different shaped magnetic fields which can be superimposed on an inhomogeneous main magnetic field. These superimposed fields shape the main magnetic field in a manner which increases the overall field homogeneity. Typically, many sets of correction coils are required. Often, as many as 10 to 20 independent sets of correction coils may be used, and each needs its own power supply to provide the correct current flow for proper operation.
Another way of providing the necessary field homogeneity is to shim the magnet passively, using pieces of iron or other ferromagnetic material, to bring an otherwise inhomogeneous field to within imaging homogeneity specifications. The iron can be applied in various forms, including small pieces called dipoles, longer pieces called bars, and rings. The bars and rings can be solid, or constructed of a plurality of dipoles. A device which uses iron dipole shims to reduce inhomogeneity in an MRI magnet is disclosed in U.S. Pat. No. 5,045,794 to Dorri et al. The Dorri patent describes a method of measuring the field inhomogeneity, measuring the effect of a defined shim placed at a plurality of defined locations, and calculating the sizes of shims to be placed at the defined locations to minimize the peak to peak inhomogeneity. This calculation is followed by placing shims as indicated by the calculation, and remeasuring the inhomogeneity. If the field is not within tolerance, the calculation is performed again, and more shims are added, or some are removed. The method disclosed by Dorri et al. involves creating a mathematical model expressing the amount of shim material used and the field inhomogeneity, and applying a single minimization procedure to this model, to attempt the simultaneous reduction of inhomogeneity throughout the DSV. Repetition of the entire procedure may be required. Thus, the method disclosed by Dorri et al. does not consider the possibility of reducing inhomogeneity in the DSV in an ordered sequence which avoids degradation of previously corrected inhomogeneities. The present invention, however, recognizes this possibility.
In light of the above, it is an object of the present invention to passively minimize inhomogeneity in the main magnetic field of an MRI magnet by properly positioning ferromagnetic shims in relation to the magnet. It is also an object of the present invention to provide a method for optimally minimizing the inhomogeneity in the DSV of a magnetic field in an ordered sequence of steps, wherein each step involves shim placement in a particular configuration designed to minimize particular contributions to the inhomogeneity. Still another object of the present invention is to provide a method for passively shimming a magnet which is easy to implement and comparatively cost effective.